Different types of carriers are known such as aircraft, ships or land vehicles whose navigation requires knowledge of position, speed and attitude (heading, roll and pitch).
A modern inertial navigation system generally includes inertial sensors such as gyrometers and accelerometers, which are integrally fastened onto the carrier (called a xe2x80x9cstrapdownxe2x80x9d system).
The gyrometers measure angular rotation and enable an analytical platform to be slaved to remain fixed in a geographical reference frame. The accelerometers measure accelerations which are projected into this analytical platform and then integrated once to supply the speed, then a second time to supply the position. Attitude is obtained by extracting 3 angles of passage from the carrier reference frame to the reference frame of the analytical platform.
The accuracy of an inertial navigation system directly depends on the errors of the inertial sensors (gyrometers and accelerometers), and more specifically on the projection of these errors in the local geographical reference frame.
For a strapdown navigation system, the projection of these errors basically depends on the trajectory of the carrier; it cannot therefore be controlled.
The accuracy of a strapdown navigation system is therefore limited by the intrinsic accuracy of these sensors. In the case of long-duration inertial navigation, the errors of position depend chiefly on the accuracy of the gyrometers.
For autonomous navigation over long periods, for several tens or even several hundreds of hours, without external means of position updating, performance basically depends on the stability of the gyrometers.
Developing special inertial components increases the cost of the inertial system without ever completely protecting the results from drift.
In order to improve the accuracy of long-duration navigation of a strapdown inertial system, we may consider using a strapdown inertial core mounted on a mechanical device enabling this core to be placed in different successive positions in order to average out the gyrometer errors projected in the local geographical reference frame.
The attitude information output from the strapdown inertial system enables the mechanical device to be directly controlled so as to place the core successively in different positions substantially fixed with respect to the local geographical reference frame.
A long-duration navigation method by means of an inertial core including a linked reference frame with axes Xm, Ym, Zm, mounted on a carrier for measuring its movements with respect to a geographical reference frame of fixed directions along three axes Xg, Yg, Zg, then comprises:
measuring actions consisting of continuously measuring the orientation of the linked reference frame in the geographical reference frame, by means of the inertial core;
positioning actions consisting of applying a sequence of 180xc2x0 turning cycles of the inertial core which each maintain the Ym axis in a direction parallel to the Yg axis, a succession of two 180xc2x0 turns around the Xm axis being preceded and followed by one 180xc2x0 turn around the Zm axis, a succession of two 180xc2x0 turns around the Zm axis being preceded and followed by one 180xc2x0 turn around the Xm axis.
The positioning actions are used to compensate for measurement errors by reversing the sign along the Ym axis at each 180xc2x0 turn, by reversing the sign along the Xm axis at each 180xc2x0 turn around the Zm axis and by reversing the sign along the Zm axis at each 180xc2x0 turn around the Xm axis.
The measuring actions carried out continuously then enable the errors to be mutually compensated on each of the Xm, Ym, Zm axes of the linked reference frame, so as to reduce the projections of errors on the Xg, Yg, Zg axes by averaging their interventions on a cycle.
Such an inertial system gives satisfactory results over a sufficiently long interval of time by averaging the defects of the inertial components. However, instantaneous measurements remain error-prone.
Although the long-term performances of such a system with a gimbals-mounted inertial core are significantly better than with a core that is fixed with respect to the carrier, as far as short-term performances are concerned, the accuracy of this system on roll, pitch and heading is limited by the accuracies of coding of the gimbals angles and of the associated processing sequence, by the deformation of the gimbals and the accuracy of their production, by the stability of a possible internal suspension of the unit supporting the inertial sensors within the inertial core. Furthermore, the rotations of the inertial core project the errors of the components in a variable way and introduce a high-frequency noise. This high-frequency noise interferes with the roll, pitch, heading and speed measurements.
In order to remedy the aforementioned drawbacks, the object of the invention is a long-duration navigation method by means of a first strapdown navigation unit generating a first state vector whose components give position, attitude and speed values of a carrier with a small instantaneous error, characterized in that it includes actions consisting of:
generating by means of a second navigation unit, a second state vector whose components give position, attitude and speed values of the said carrier with a small long-term error;
combining the first and second state vector to obtain an error observation vector;
generating an error estimate vector from the error observation vector, by means of a statistical filter;
combining the error estimate vector thus obtained with the first state vector so as to obtain an estimated vector of the carrier position, attitude and speed values with a small instantaneous error and a small long-term error.
The object of the invention is also a navigation device for obtaining an estimated vector of position, attitude and or speed values of a carrier with a small instantaneous error and a small long-term error, characterized in that it includes:
a first strapdown navigation unit set up to generate a first state vector whose components give position, attitude and speed values of a carrier with a small instantaneous error;
a second navigation unit set up to generate a second state vector whose components give position, attitude and speed values of the said carrier with a small long-term error;
a statistical filter set up for combining at the input the first and second state vector so as to generate at the output of the said filter an error estimate vector for obtaining an estimated vector of the carrier position, attitude and speed values with a small instantaneous error and with a small long-term error.